Ambush Feeders Research Articles

Prey Detection and Prey Capture in Copepod Nauplii

Copepod nauplii are either ambush feeders that feed on motile prey or they produce a feeding current that entrains prey cells. It is unclear how ambush and feeding-current feeding nauplii perceive and capture prey. Attack jumps in ambush feeding nauplii should not be feasible at low Reynolds numbers due to the thick viscous boundary layer surrounding the attacking nauplius. We use high-speed video to describe the detection and capture of phytoplankton prey by the nauplii of two ambush feeding species (Acartia tonsa and Oithona davisae) and by the nauplii of one feeding-current feeding species (Temora longicornis). We demonstrate that the ambush feeders both detect motile prey remotely. Prey detection elicits an attack jump, but the jump is not directly towards the prey, such as has been described for adult copepods. Rather, the nauplius jumps past the prey and sets up an intermittent feeding current that pulls in the prey from behind towards the mouth. The feeding-current feeding nauplius detects prey arriving in the feeding current but only when the prey is intercepted by the setae on the feeding appendages. This elicits an altered motion pattern of the feeding appendages that draws in the prey.

Danger of zooplankton feeding: the fluid signal generated by ambush-feeding copepods

Zooplankton feed in any of three ways: they generate a feeding current while hovering, cruise through the water or are ambush feeders. Each mode generates different hydrodynamic disturbances and hence exposes the grazers differently to mechanosensory predators. Ambush feeders sink slowly and therefore perform occasional upward repositioning jumps. We quantified the fluid disturbance generated by repositioning jumps in a millimetre-sized copepod (Re ∼ 40). The kick of the swimming legs generates a viscous vortex ring in the wake; another ring of similar intensity but opposite rotation is formed around the decelerating copepod. A simple analytical model, that of an impulsive point force, properly describes the observed flow field as a function of the momentum of the copepod, including the translation of the vortex and its spatial extension and temporal decay. We show that the time-averaged fluid signal and the consequent predation risk is much less for an ambush-feeding than a cruising or hovering copepod for small individuals, while the reverse is true for individuals larger than about 1 mm. This makes inefficient ambush feeding feasible in small copepods, and is consistent with the observation that ambush-feeding copepods in the ocean are all small, while larger species invariably use hovering or cruising feeding strategies.

Mechanisms and feasibility of prey capture in ambush-feeding zooplankton

Many marine zooplankters, particularly among copepods, are "ambush feeders" that passively wait for their prey and capture them by fast surprise attacks. This strategy must be very demanding in terms of muscle power and sensing capabilities, but the detailed mechanisms of the attacks are unknown. Using high-speed video we describe how copepods perform spectacular attacks by precision maneuvering during a rapid jump. We show that the flow created by the attacking copepod is so small that the prey is not pushed away, and that the attacks are feasible because of their high velocity (approximately 100 mm x s(-1)) and short duration (few ms), which leaves the prey no time for escape. Simulations and analytical estimates show that the viscous boundary layer that develops around the attacking copepod is thin at the time of prey capture and that the flow around the prey is small and remains potential flow. Although ambush feeding is highly successful as a feeding strategy in the plankton, we argue that power requirements for acceleration and the hydrodynamic constraints restrict the strategy to larger (> 0.25 mm), muscular forms with well-developed prey perception capabilities. The smallest of the examined species is close to this size limit and, in contrast to the larger species, uses its largest possible jump velocity for such attacks. The special requirements to ambush feeders with such attacks may explain why this strategy has evolved to perfection only a few times among planktonic suspension feeders (few copepod families and chaetognaths).

Optimal swimming strategies in mate-searching pelagic copepods

Male copepods must swim to find females, but swimming increases the risk of meeting predators and is expensive in terms of energy expenditure. Here I address the trade-offs between gains and risks and the question of how much and how fast to swim using simple models that optimise the number of lifetime mate encounters. Radically different swimming strategies are predicted for different feeding behaviours, and these predictions are tested experimentally using representative species. In general, male swimming speeds and the difference in swimming speeds between the genders are predicted and observed to increase with increasing conflict between mate searching and feeding. It is high in ambush feeders, where searching (swimming) and feeding are mutually exclusive and low in species, where the matured males do not feed at all. Ambush feeding males alternate between stationary ambush feeding and rapid search swimming. Swimming speed and the fraction of time spent searching increase with food availability, as predicted. This response is different from the pattern in other feeding types. The swimming speeds of non-feeding males are predicted and observed to be independent of the magnitude of their energy storage and to scale with the square root of body length in contrast to the proportionality scaling in feeding copepods. Suspension feeding males may search and feed at the same time, but feeding is more efficient when hovering than when cruising. Therefore, females should mainly be hovering and males cruising, which is confirmed by observations. Differences in swimming behaviour between genders and feeding types imply different mortality rates and predict well the observed patterns in population sex ratios. Sex ratios may become so female-biased that male abundances limit population growth, demonstrating that behaviours that are optimal to the individual may be suboptimal to the population.

Mate finding, mating, and population dynamics in a planktonic copepod Oithona davisae: There are too few males

Three‐dimensional (3D) video observations of free‐swimming copepods revealed the mechanisms of mate finding in the cyclopoid copepod Oithona davisae (cephalothorax length 0.33 mm). The females are ambush feeders and spend almost all of their time sinking slowly through the water, interrupted by 0.5–1‐mm‐long jumps every few seconds. The sink—jump behavior distributes produced pheromones along an intermittent trail. Males spend about half of their time ambush feeding, and half of their time cruising rapidly (50 body lengths s−1), searching for female signals. On encountering a pheromone patch, the male starts circling rapidly (up to >100 body lengths s−1) around the patch, apparently to find subsequent patches and, eventually, the female. The pursuit may last several tens of seconds, and ends by the male capturing the female or losing the track. Observed reaction distances and male motility patterns were combined with a simple encounter model to estimate a matesearch volume rate of 4.4 L d−1. The males have a rather limited mating capacity, 0.9 females male−1 d−1, even when mate‐encounter rate is not limiting. Field observations of abundances, combined with estimates of mortality and mate‐search and mating capacities, predict that only approximately one‐third of the females should be fertilized. This is consistent with field observations, and suggests that population growth in this species is severely constrained by fertilization limitation. Fertilization limitation may apply more generally to populations of pelagic copepods, and may partly account for the frequent observation that female copepods produce sterile eggs, even during periods of a plentiful food supply.

A new species of pitviper of the genus Popeia (Squamata: Viperidae) from Pulau Tioman, Pahang, West Malaysia

We describe a new species of pitviper Popeia buniana from Pulau Tioman, Pahang State, West Malaysia based on it having unique aspects of scalation, body proportions, and color pattern. Its long, thin body is in contrast with the shorter, more robust bodies of other arboreal pitvipers, suggesting it may be an active forager rather than an ambush feeder. Its close relationship to P. barati and P. sabahi of Sumatra and Borneo, respectively, rather than with species of the adjacent Malay Peninsula is in accord with biogeographic patterns of other flora and fauna endemic to the islands of the Outer Arc of the Seribuat Archipelago.

Turbulence and feeding behaviour affect the vertical distributions of Oithona similis and Microsetella norwegica

The small copepods Oithona similis and Microsetella norwegica are often numerically abundant and widely distributed, but the factors controlling their vertical distributions and role in carbon cycling are yet unknown. Here we examined the vertical distributions of copepods during spring and summer in the Skagerrak and during autumn in the North Sea with respect to different physiochemical factors including turbulent dissipation rate. The ambush feeder O. similis numeri- cally dominated the copepod community; they were located in the layers with high microzooplank- ton abundance. M. norwegica were dominant in the Skagerrak and were observed within or just below the pycnocline; they are assumed to feed on sinking detrital aggregates. Both copepods use remote detection of either hydromechanical (O. similis) or chemical signals (M. norwegica) generated by the prey and both species migrated to deeper depths in response to elevated surface turbulence. The potential effect of turbulence on both types of feeding is theoretically shown to be negative and we suggest a turbulent dissipation rate in the range 10 -7 to 10 -6 m 2 s -3 as a threshold triggering the observed avoidance responses.

Predatory and suspension feeding of the copepod Acartia tonsa in turbulent environments

The copepod Acartia tonsa exhibits 2 different feeding modes: when feeding on small phytoplankton cells it sets up a feeding current and acts as a suspension feeder; when feeding on motile prey it acts as an ambush feeder. We examined experimentally the effects of small-scale turbulence on feeding rates in these 2 modes. The different feeding behaviours were triggered by offering the copepods diatoms Thalassiosira weissfloqii and ciliates Strombidium sulcatum, respectively. Turbulence at 5 different intensities (energy dissipation rate, E, between 4 X I O ~ and 3.7 X 10' cm2 s ~ ) was generated by an oscillating grid. In ambush feeding mode, low (realistic) intensities of turbulence (E = 1 0 ' ~ to 1 0 ~ cm2 s ~ ) enhanced clearance rates by up to a factor of 4 above those observed in calm water Higher intensities of turbulence ( E = 10-' to 10' cm2 s ~ ) resulted in a depression of clearance rates, although the rates were still significantly higher than those observed in calm water. The depression of clearance rates at high turbulence intensities was due partly to a decline in capture success, but mainly to a decrease in reactive distance, because turbulence interferes with prey perception by disturbing the hydrodynamical signal generated by motile prey. The negative effects were evident only at turbulence intensities exceeding those normally encountered by A tonsa in its natural habitat. In suspension feeding mode, low intensities of ambient turbulence (E = I O ~ to 10-2 cm2 s- had negligible effects on clearance rates, while at higher turbulence intensities (E = 10-I to 10' cm2 s ~ ) we observed a negatlve effect (depression of clearance rate). The negative effects become evident when ambient turbulent fluid shear approaches the maximum shear rate of the copepod's feeding current, and we hypothesize that at these intensities the feeding current is eroded. Again the negative effects were observed only at turbulence intensities higher than those typically experienced by A. tonsa in the sea. The differential response to turbulence of the 2 feeding behaviours, including the negative effects, were accurately predicted by encounter rate and feeding behaviour models proposed by korboe & Saiz (1995; Mar Ecol Prog Ser 122:135-145). Because feeding behaviour is specific to the prey (phytoplankton vs mot~le prey), and because ambush-mode feeding is much more dependent on turbulence than suspensionmode feeding, our findings suggest that prey selection in A. tonsa may be partly governed by turbulence in the ocean. This may explain why rnicrozooplankton at times dominates the diet of A. tonsa and other copepods, even though it is numerically scarce relative to phytoplankton in the environment.

The specialized diet of Harpagifer bispinis:

The diet of Harpagifer bispinis (Pisces: Nototheniidae) from two localities of the South Shetland Archipelago was studied. Simultaneous to the capture of H. bispinis and at the same sites the availability of food was considered, and amphipod diversity was compared with the density of Harpagifer. It was found that three quarters of the fish fed only on amphipods (mainly Gondogeneia antarctica) and for the rest amphipods were also the main component, even when other prey species were available. The high selectivity of G. antarctica is due to its high mobility and to the fact that Harpagifer is an ambush feeder. At different predator densities the amphipod fraction of the community appears to be highly modified by the predator both numerically and in species evenness. We postulate that Harpagifer can be a key species in structuring the mobile epibenthic community, even when this environment is subject to strong physical stress.